LED BAR TACHOMETER



Below is the bill of materials compiled with the Advanced Schematic; somehow it doesn't appear sorted in any reasonable order; unfortunately, I don't have time to manually rearrange all the entries. The parts listed below are oriented on breadboard assembly; if you feel comfortable with surface-mount technology, check with Digikey for parts in SMD package. There is also a higher-resolution screen capture of the circuit diagram.
 
Part Type Designator Footprint DigiKey part # Price, US$ Description
0.047mF C2   P4251-ND 1.23 suppresses multiple counts from voltage overshoots
0.047mF C3   P4251-ND 1.23 storage capacitor - increasing its value decreases the RPM range
1.0mF C4   P2105-ND 0.28 smoothes the tach readout. Increasing 
its value decreases ripple
1.0mF 50V C1   P2073-ND 0.51 isolates the tach input from +12V DC on the coil
100k R3 0.125W  CT9P104-ND 1.65 Tunable: cap C3 discharges through it. Increasing its value decreases RPM range
150k R1 0.5W  150KH-ND 0.27(5) reduces the input voltage on pin 1 of LM2907 chip. Better to mount it in-line
1N4001 D22   1N4001MSCT
-ND
0.43 protects LM2907 input from overvoltage 
(clamps it to +6V rail)
1N4001 D21   1N4001MSCT
-ND
0.43 protects LM2907 input from overvoltage 
(clamps it to ground)
1N4001 D23   1N4001MSCT
-ND
0.43 protects entire circuit from reverse-polarity connection
2.2k R6 0.125W  2.2KEBK-ND 0.28(5) determines the current through LEDs: 
I_led = 10 * 1.2V / R6
2.2k R5 0.125W  2.2KEBK-ND 0.28(5) determines the current through LEDs: 
I_led = 10 * 1.2V / R5
33k R2 0.125W  33KEBK-ND 0.28(5) provides DC coupling of LM2907 input to the ground
33k R4 0.125W  33KEBK-ND 0.28(5) loading resistor for open emitter output of LM2907 (pin 4)
7806 U4 TO220 NJM7806FA
-ND
0.65 6V DC voltage regulator; any chip from 7805, 7806, or 7808 family can be used. 
LED D18   67-1045-ND 0.26 Amber LED - getting high in RPM
LED D17   67-1045-ND 0.26 Amber LED - getting high in RPM
LED D20   67-1047-ND 1.32(10) Red LED - redline 1 - OOPS
LED D19   67-1047-ND 1.32(10) Red LED - redline 2 - OOPS
LED D6   67-1046-ND 1.23(10) Green LED
LED D5   67-1046-ND 1.23(10) Green LED
LED D8   67-1050-ND 1.41(10) Yellow LED - 2k RPM
LED D7   67-1046-ND 1.23(10) Green LED
LED D2   67-1045-ND 0.26 Amber LED - getting low in RPM
LED D1   67-1047-ND 1.32(10) Red LED - about to stall
LED D4   67-1050-ND 1.41(10) Yellow LED - 1k RPM
LED D3   67-1050-ND 1.41(10) Yellow LED
LED D14   67-1046-ND 1.23(10) Green LED
LED D13   67-1046-ND 1.23(10) Green LED
LED D16   67-1050-ND 1.41(10) Yellow LED - 4k RPM
LED D15   67-1050-ND 1.41(10) Yellow LED
LED D12   67-1050-ND 1.41(10) Yellow LED - 3k RPM
LED D9   67-1046-ND 1.23(10) Green LED
LED D10   67-1046-ND 1.23(10) Green LED
LED D11   67-1046-ND 1.23(10) Green LED
LM2907-8N U1 DIP8 LM2907N-8-ND 2.28 Frequency - to - Voltage converter chip
LM3914N U2 DIP16 LM3914N-ND 3.29 LED bar / dot driver chip
LM3914N U3 DIP16 LM3914N-ND 3.29 LED bar / dot driver chip
Breadboard ~5.00 Check your local Radio Shack
Total cost 26.27



Note: some discrepancies may appear between the GIF picture and the table. Most likely, the table contains the correct information. Well, a usual disclaimer - this circuit is not guaranteed to work (though it was copied from a working breadboard version), nor it is guaranteed not to fry your electrical system. Please use your common sense anytime you mess with automotive electrics - I've had smokes despite many years of experience in electronics!

Last Updated on 12/24/2012
By Peter Matusov
Email me with any questions or comments - although I very rarely check this mailbox